1. Field of the Invention
This invention relates to metal oxides containing pentavalent ruthenium and having a perovskite-type crystal structure, and to their use as emission control catalysts.
2. Description of the Prior Art
It is known that Ru and Ru-containing compounds are good reduction catalysts, and thus their use to reduce NO.sub.x to N.sub.2 in exhaust gases from internal combustion engines has been proposed. However, Ru is normally volatile at the high temperatures encountered in these exhaust gases.
In Platinum Metals Review, vol. 18, no. 1, p. 2 (1974), Shelef et al. teach stabilizing Ru against loss by volatilization by incorporating Ru oxide in a compound with basic oxides such as BaRuO.sub.3.
In a dissertation submitted to the University of Connecticut in 1965, P. C. Donohue disclosed hexagonal close packed structures of alkaline earth ruthenium oxides of the formula Ba(RuM)O.sub.3 in which M is various metals, some of said compounds having perovskite-type crystal structures and certain others having ruthenium in the pentavalent state. This subject is further disclosed by Donohue et al. in Inorganic Chemistry, vol. 5, no. 3, pp. 339-342 (1966). In this article compounds in which M is Zr, Mn, Ni, and Mg are described.